The news gives Last.fm’s main competitor, the uber-popular Pandora, a leg up, since it still offers free, ad-supported, music streaming apps.

Even though it relies on ads for its website, Last.fm has found it impractical to pursue that strategy for mobile and home entertainment devices, according to vice president of product Matthew Hawn. So now users will have to cough up $3 a month if they want Last.fm music on those devices, including the iPhone, iPad, and Android phones.

The company will continue to offer free music streaming on its website in the US, UK and Germany, as well as to Xbox Live and Windows Phone 7 users in the US and UK. Hawn points out that other aspects of its apps, including scrobbling (the term it uses for liking or disliking music), music and event recommendations, and more, will still be available for free.

I don’t think the pricing will be much of a problem for hardcore users of Last.fm — especially since they can pay $3 monthly, instead of one lump annual sum. (Pandora offers its own premium service, Pandora One, for $36 a year.) But it could be difficult for the company to entice new users when they can get Pandora streaming music for free on their phones.